Pidgin is a multi-protocol chat-client which supports AIM, Google Talk, Jabber/XMPP and many more. Pidgin is available for Mac, Linux and Windows.
Download the relevant installation package at https://pidgin.im/.
Install the software and then configure GWDG's XMPP service according to the directions.
Linux and Mac OS X
The easiest way to install Pidgin is using the repositories of you package manager. OS X users should have a look at Macports. Linux users should use the software manager of their distribution.
Pidgin starts on first boot with a simple account-creation-wizard that asks for all necessary credentials to connect to an instant messaging service. Klick on “Add” to configure a new service.
To use the GWDG jabber-service please choose “XMPP” as protocol-type. Add your GWDG-credentials (username and password). As domain use jabber.gwdg.de. See also the overview of the server data.
After connecting to the GWDG jabber server the list of contacts is being loaded. This process can take up to one minute.